As demand grows to transmit ever more information through optical fiber, there is a growing need for devices capable of performing signal processing in the optical domain. These devices are typically fabricated as planar lightwave circuits (PLCs), that is, optical wave guiding structures deposited on a planar substrate. A PLC design that is particularly useful in many applications (such as add/drop multiplexing, dispersion compensation, and multi-channel filtering) is the Resonant Coupler (RC) 1-4!.
The RC is a signal processing circuit structure in which light is coupled back and forth between two wave-guides at a series of directional couplers. Between each neighboring pair of directional couplers, the light in one wave-guide is delayed relative to the other (see FIG. 1). The same optical delay length (to within a wavelength) is introduced between each neighboring pair of directional couplers. The RC is a form of digital delay filter.
Jinguji and Kawachi 2! have demonstrated that one can generate an arbitrary Nth order finite impulse response (FIR) filter with an N stage RC. FIG. 1 shows a schematic of a conventional resonant directional coupler, an N stage RC, defined to consist of N directional couplers separated by N-1 delay lengths. To obtain the desired impulse response, one chooses an appropriate distribution of coupling strengths for the directional couplers and an appropriate distribution of relative phases induced by the delay stages.